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[nova][ptg] Allow compute nodes to use DISK_GB from shared storage RP by using aggregate relationship


spec: https://review.opendev.org/#/c/650188/

Agreements from the room[1]:
* new config option 'using_shared_disk_provider' (just an example name) 
on the compute level to ignore the DISK_GB reporting from the driver
* new config option 'sharing_disk_aggregate' (just an example name) on 
the compute level to tell nova compute what is the UUID of the 
placement aggregate that contains the sharing DISK_GB providers in 
placement.
* the "using_shared_disk_provider" flag necessarly has to be explicit 
since if not, it would be a chicken-and-egg problem on a greenfields 
install as the shared RP wouldn't be created
* deployer needs to create the sharing disk RP and report inventory / 
traits on it
* deployer needs to define the placement aggregate and add the sharing 
disk RP into it
* when compute restarts and sees that 'using_shared_disk_provider' = 
True in the config, it adds the its compute RP to the aggregate defined 
in 'sharing_disk_aggregate' Then if it sees that the root RP still has 
DISK_GB inventory then trigger a reshape
* os-hypervisor API response (in a new microversion) will have a link 
to the sharing disk RP if the compute is so configured.

TODO:
* tpatil to update the spec

[1] https://etherpad.openstack.org/p/nova-shanghai-ptg